The other day my wife and I were in one of the big box stores and we overheard a conversation between a customer and a salesman about the customer's iPod.
It seems that the gentleman was in town for an extended stay, had access to a computer, and wanted to add a few new tunes to his iPod. The salesman proceeded to tell this poor guy that if he tried it on a computer that didn't have his personal iTunes library on it he would wipe all but the new songs off of his iPod. He suggested that the only way to do it would be to either go home and copy his library onto a thumb drive, which he could then bring back and load onto another machine, or have somebody do it for him and mail it to him.
It took every fiber of my being to keep my mouth shut. The evil look from my wife helped also. She hates it when I stick my nose in where it doesn't belong.
What I hate is misinformation.
Now it's true that if you autosync your iPod with an empty library, you end up with an empty iPod. And it's also true that having someone go to your house and copy your library and then mail it to you is a valid solution, but there are some safe and much less painlful ways to accomplish the same thing.
First and foremost, when you connect your iPod to a new computer iTunes always asks if you want to sync it. Just say no (or more accurately, just hit cancel). Once you tell it not to autosync, you can open up the iPod control panel and set it for manual sync. At this point your iPod's onboard library is safe and you then have a number of options.
You can proceed to purchase or rip music into iTunes and manually add it to the iPod. Remember to leave it in manual mode when you're done, otherwise you'll loose your new music the next time you autosync back home. You could then use the thumb drive trick or burn a CD to transport the new music back home.
You can also use either iTunes or a number of third party solutions to copy your songs from the iPod onto the new computer to recreate your library. Starting in one of the early versions of iTunes 7 there is a command under the File tab "Transfer purchases from iPod" that will allow you to move iTunes purchased music only to the new library. This is a fine option if all you have is iTunes purchased music, stick with the third party solutions if you don't.
Here's a great article on iPod to computer transfer from iLounge. There's a really nice comparison chart of third party solutions and some tips and warnings. Be sure to read it before you try anything.
Note: Not all third party transfer software will work with the new iPod touch and iPhone as they have a different synching protocol than the standard iPods. Some that do are: CopyTrans, iPod Access, and TouchCopy.